1. Field of the Invention
The present invention relates to an optical fiber apparatus having a fusion-spliced portion in which two optical fibers are fusion-spliced, and a fabrication method thereof.
2. Related Background Art
As methods of cascade connection between two optical fibers, there are connector splice and fusion splice. Among these, the fusion splice is a method of aligning two optical fibers each other and heating end faces of the respective fibers to cause fusion thereof, and has the advantage of low splice loss. When two optical fibers of mutually different mode field diameters are spliced by fusion splice, the splice loss increases because of the difference between the mode field diameters. The splice loss can be reduced based on decrease in the difference between the mode field diameters by diffusion of an additive (for example, Ge or F) by heating at a predetermined temperature after the fusion splice. On the other hand, however, the fusion splice had the problem that the mechanical strength of the fusion-spliced portion was lower than that of the other portions of the optical fibers. Conventionally, in order to reinforce the fusion-spliced portion in the optical fiber apparatus including it, a steel wire was placed along the fusion-spliced portion and the whole of these were coated with resin.
The reinforcing method of the fusion-spliced portion with the steel wire as described above, however, has the following problems. Namely, the outside diameter of the reinforced portion (the portion along with the steel wire) including the fusion-spliced portion becomes larger than that of the other portions of the optical fibers. With increase in the outside diameter of the reinforced portion along with the steel wire, stress will be exerted on the optical fibers around this reinforced portion during the procedure of forming a cable from the two optical fibers spliced in cascade connection including this reinforced portion, during the procedure of forming a module by winding the fibers around a bobbin, or during other mounting procedures, so that it will raise the risk of fracture of the optical fibers or increase loss of light propagating in the optical fibers. It also becomes harder to bend this reinforced portion during the procedure of winding the two optical fibers including the reinforced portion around the bobbin. For the optical fiber apparatus having the fusion-spliced portion, as described, it is sometimes undesirable to employ the reinforcement of the fusion-spliced portion with the steel wire, depending upon mounting forms or uses.
The present invention has been accomplished to solve the above problems and an object of the present invention is thus to provide optical fiber apparatus that secures the sufficient mechanical strength of the fusion-spliced portion without use of the reinforcement with the steel wire, and a method of fabricating the optical fiber apparatus.
A fabrication method of optical fiber apparatus according to the present invention is a method of fabricating optical fiber apparatus having a fusion-spliced portion in which respective end faces of two optical fibers are spliced each other by fusion splice, which comprises a fusion step of heating the end faces of the two optical fibers to cause fusion thereof, thereby forming the fusion-spliced portion, an additive-diffusing step of diffusing an additive added in the two optical fibers, by a heating treatment of the fusion-spliced portion at a first temperature of not less than 800xc2x0 C. nor more than 1500xc2x0 C., and a thermal-strain-removing step of removing thermal strain by a heating treatment of a wider region than heated regions in the fusion step and in the additive-diffusing step of the fusion-spliced portion, at a second temperature being not less than 500xc2x0 C. nor more than 1200xc2x0 C. and being lower than the first temperature, after the additive-diffusing step.
According to this fabrication method of optical fiber apparatus, in the fusion step the end faces of the two optical fibers are heated to cause the fusion thereof to form the fusion-spliced portion and in the additive-diffusing step thereafter the fusion-spliced portion is subjected to the heating treatment at the first temperature of not less than 800xc2x0 C. nor more than 1500xc2x0 C. to diffuse the additive added in the two optical fibers near the fusion-spliced portion. In the thermal-strain-removing step after the additive-diffusing step, the thermal strain is removed in and around the fusion-spliced portion by the heating treatment of the wider region than the heated regions in the fusion step and in the additive-diffusing step of the fusion-spliced portion, at the second temperature being not less than 500xc2x0 C. nor more than 1200xc2x0 C. and being lower than the first temperature, and thereafter the region is cooled.
Even in the case of two optical fibers of mutually different mode field diameters being spliced each other by fusion splice, the splice loss can be reduced based on decrease in the difference between the mode field diameters, by diffusing the additive by the heating treatment at the first temperature in the additive-diffusing step. The thermal strain appears near the fusion-spliced portion in the fusion step and in the additive-diffusing step, but this thermal strain is removed in the thermal-strain-removing step of heating the wider region than the heated regions in the fusion step and in the additive-diffusing step of the fusion-spliced portion, at the second temperature being not less than 500xc2x0 C. nor more than 1200xc2x0 C. and being lower than the first temperature, whereby the sufficient mechanical strength is ensured in and around the fusion-spliced portion of optical fiber apparatus. Here the xe2x80x9cheated regionxe2x80x9d stated in the present invention means a region in which the temperature becomes 500xc2x0 C. and higher during heating of the optical fibers. The removed state of thermal strain is also maintained during the cooling after the thermal-strain-removing step, so that the satisfactory mechanical strength is ensured in and around the fusion-spliced portion of optical fiber apparatus. The cooling rate is preferably not more than 4000xc2x0 C./min.
The fabrication method of optical fiber apparatus according to the present invention is characterized in that in the thermal-strain-removing step the heating treatment is carried out by arc discharge, by flame generated with supply of inflammable gas and oxygen gas to a burner, or by a heater. In either of these cases, the thermal-strain-removing step is carried out to perform the heating treatment in and around the fusion-spliced portion of optical fiber apparatus at the appropriate temperature, so as to remove the thermal strain caused in the fusion step. The heating by arc discharge is preferred, because a fusion splicing machine can be used as it is. The heating by the burner is preferred, because it permits distribution heating in and around the fusion-spliced portion of optical fiber apparatus and use of a compact heating device. The heating by the heater is preferred, because it permits the distribution heating in and around the fusion-spliced portion of optical fiber apparatus and keeps a heating atmosphere clean.
The fabrication method of optical fiber apparatus according to the present invention is characterized in that in the thermal-strain-removing step the above heating treatment is carried out while moving a heat source relative to the two optical fibers along the longitudinal direction thereof. In this case, the thermal strain in the heated range during the additive-diffusing step can be properly removed in the thermal-strain-removing step.
The fabrication method of optical fiber apparatus according to the present invention is characterized in that in the thermal-strain-removing step a temperature gradient along the longitudinal direction of the two optical fibers is not more than 500xc2x0 C./mm. In this case, since the temperature gradient is small, the thermal strain is adequately removed.
An optical fiber apparatus according to the present invention is characterized by being fabricated by the aforementioned fabrication method of optical fiber apparatus according to the present invention. Since the thermal strain caused in the fusion step is removed from the optical fiber apparatus in the thermal-strain-removing step, the satisfactory mechanical strength is ensured in and around the fusion-spliced portion without use of the reinforcement with the steel wire. Even in the case of optical fiber apparatus in which two optical fibers of mutually different mode field diameters are spliced each other by fusion splice, since the additive is diffused by the heating treatment at the first temperature in the additive-diffusing step, the difference is small between the mode field diameters and the splice loss is thus small.
The optical fiber apparatus stated herein means one having the fusion-spliced portion in which two or more optical fibers are spliced each other by fusion splice, and involves optical fiber transmission lines to be laid (e.g., a dispersion compensating optical fiber+a single-mode optical fiber), modules on bobbins (e.g., a dispersion compensating optical fiber+a single-mode optical fiber, and an Er-doped optical fiber for optical amplification+a single-mode optical fiber), and other forms.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.